In general, there are known electric power tools that have a speed changing function with a view to enhance work efficiency (see, e.g., Japanese Patent Laid-open Publication No. 63-101545).
One example of the electric power tools is shown in FIG. 15. This electric power tool includes a motor 101 as a driving power source, a speed reducer unit 102 for delivering the rotational power of the motor 101 at a reduced speed, a drive unit (not shown) for delivering the rotational power of the speed reducer unit 102 to a tip end tool, a resin-made housing 104 provided with a handle portion 104a and arranged to contain the motor 101 and the speed reducer unit 102 therein, an operation lever 105 and a shift unit 105a, both of which serve as a speed changing mechanism for changing the gear reduction ratio of the speed reducer unit 102, the operation lever 105 being arranged in a position where it can be operated outside the housing 104, a power switch 106 installed in the handle portion 104a for switching on and off the power supply of the motor 101, and a battery pack 107 engaged with the housing 104 for supplying electric power to the motor 101.
As shown in FIGS. 16A and 16B, the operation lever 105 is designed to convert the tool operation state to a low-speed high-torque state in a high load condition (when the work load is heavy) but to a high-speed low-torque state in a low load condition (when the work load is light). This makes it possible for the electric power tool to perform a desired tightening task depending on the work load, thereby increasing the efficiency of work.
In case the work load varies in the midst of work, the operation lever 105 may be operated during the work to change the gear reduction ratio. This may sometimes cause trouble to the electric power tool. More specifically, if the gear reduction ratio is changed with the operation lever 105 during the course of work, namely if the gear 102a of the speed reducer unit 102 is shifted when in rotation, the mutually engageable gears may make contact with each other during their rotation and may be worn or damaged. This may be a cause of trouble in the electric power tool. The conventional solution to this problem is to increase the strength of gears, thereby preventing occurrence of trouble. In this case, however, the gears need to be made of high strength metal or formed into a big size, which leads to a problem of high cost and increased weight.